The present invention relates in general to an encoding and decoding device and method for playing an RF signal loaded from a record medium on which is recorded information utilizing an RLL (Run Length Limited) code, decoding this signal based on at least one reference level and issuing channel bit data, and in particular relates to an encoding and decoding device and method for selecting bits with a high error probability based on appended information when locations exist in the channel bit data where the conditions for identical minimum continuous length or maximum continuous length symbols are not satisfied, correcting the selected bit and issuing channel bit data satisfying the conditions for identical minimum continuous length or maximum continuous length symbols.
When sending data to a specified path, for instance when recording on a record medium such as magnetic disks, optical disks, or optical magnetic disks, the data is modulated to match the transmission and record media. One such modulation method is known as block encoding. In block encoding, blocks are formed in units composed of data strings of m.times.i bits (hereafter called data terms) and these data terms are converted into coded terms composed of n.times.i bits in compliance with applicable encoding rules. These block codes are fixed length codes when i=1 and when more than one element is chosen for i, in other words when i=2 or more, the maximum i when i max=r is a variable length code. These block encoded symbols are shown as variable length symbols (d, k; m, n; r). Here i is the bound length and r is the maximum bound length. Also, the symbols d and k are a symbol "1" and the other symbol being "0" placed consecutively between the "1" and "1" symbols and representing respectively the minimum continuous quantity and maximum continuous quantity in the code train.
As a specific example, the modulation system for the compact disc (CD) is explained next. The compact disc uses Eight to Fourteen Modulation or (EFM). In this modulation system, an eight bit data term is pattern-converted into a 14 bit code term (channel bit) after which a 3 bit machine bit is added in order to reduce the DC components resulting from EFM modulation, the result is recorded on an NRZI modulated disc. In this case, 8 bits are converted to 14 bits and machine bits are also added to satisfy the condition that the minimum continuous quantity (specified length) is 2 for the "0" (zeroes) between the consecutive "1" and "1" within the code train and the maximum continuous quantity (specified length) for "0" is 10. The parameters for the variable length codes (d, k; m, n; r) due to this EFM modulation are (2, 10; 8, 17; 1). When an interval T is set for the channel bit string (record waveform) interval, the minimum inversion interval T min (specified length) is 3 (=2+1) T. Further, the maximum inversion interval T max (specified length) is 11 (=10+1) T. Also, when T data is set as the data interval for the data string, the detection window width Tw is expressed as (m/n).times.T data, The value for Tw becomes 0.47 (=8/17) T data.
Also, after NRZI modulation for EFM modulated record, the identical symbol d' for minimum continuous length is 3 (=d+1=2+1) and the identical symbol for maximum continuous length k' after NRZI modulation is 11 (=k+1=10+1).
If the bits are shrunk in the compact disk in the direction of linear velocity, the recording density can be increased. The length of the minimum bit corresponding to the minimum inversion interval T min will become smaller. However bit detection becomes difficult when the minimum bit becomes too much smaller than the spot size of the laser beam causing errors to occur.
The error rate becomes even worse when skew occurs on the disk playback surface. Disk skew is a tilting of the laser beam axis versus the disk and is grouped into tangential skew within the surface parallel to disk motion, and radial skew perpendicular to the disk surface. Among these two types of skew, in tangential skew the adverse effects on error rate occur comparatively quickly. Therefore this type of skew is a factor in reducing the allowable error rate margin in the system design.
On investigating the error distribution for identical symbol continuous length, it was found that errors causing tangential skew mainly occurred when the identical symbol continuous length was short. In other words, it was found the error rate worsened because the T min (d') length was decoded into the T min-1 (d'-1) length. For instance, when skew occurred in the tangential direction in the EMF modulation system, and the bit interval of the record waveform is set to T, many errors will occur because the minimum inversion interval T min which is 3T (specified length) is decoded as the even shorter interval 2T (in violation of specified length).
In optical disks on the other hand, a certain margin for an asymmetric disk is allowed for in the manufacture however a playback waveform that deviated asymmetrically up and down versus the center level must be taken into account.
In the conventional art, the Viterbi decode method is known as a method for compensating with signal processing for poor error rates. In the Viterbi decode method, encoding errors are reduced, the path that is shortest geometrical distance searched and the most likely value by discarding poor probability path in what amounts to a simplified decoding method. The Viterbi decode method also allows adding an algorithm for correcting the minimum inversion interval T min.
However, the Viterbi decode method has the drawback of requiring a complex circuit making the required hardware of a larger scale. Further, the asymmetry must be eliminated in order to decode with this method. In optical disks the system allowing for a certain asymmetry makes optimization of this asymmetry necessary making this circuit even more complicated. Upon which this applicant proposed for instance in Tokugan-Hei 8-22530, a Run-Detector method as a method using a simpler circuit for performing correction by signal processing of bad error rates.
A block diagram showing the encoding/decoding device for this proposal is shown in FIG. 24. The waveform equalizer1 in FIG. 24 forms an analog signal waveform from the signal that was input. The PLL circuit 2 generates a bit clock pulse based on the analog waveform formed by the equalizer1. An A/D converter 3 changes the analog signal into the specified digital signal. This A/D converted digital data is then converted to a 1 or 0 bit string (binary data) based on the center level (zero) by the comparator 4.
The error length detection 5 an interval as an error length if this interval is shorter than minimum interval T min which is the specified length. For instance, if the EFM modulation code symbols (d, k) are used, when the record waveform bit interval is set as T, a T min of 3T (specified length) which was mistakenly converted to 2T (error length) as a bit string of binary data will be detected by the detector 5. Next, based on the position where the error length was detected, the position correction detector 6 corrects the bit immediately prior to or immediately after the error bit string matching the error length. In other words, the size of the signal level output from the A/D converter 3 for the bit immediately prior to or immediately after the error bit string having the error length 2T is compared and a bit with a signal size near zero level is specified as the error bit correction position. The correction processor 7 corrects the bit specified in the channel bit string as the correction position and then outputs this channel bit string data after correction is complete.
However in the method proposed as shown in FIG. 24, correction is performed by installing an A/D converter circuit in the equipment and utilizing the playback signal level (information on direction of amplitude). Therefore in systems where an A/D converter circuit is basically unnecessary an A/D converter circuit must be installed for error correction, thus having drawbacks such as making the circuit configuration more complex and increasing the cost. In view of the above problems, this invention will provide signal processing of the signal containing the bad error rate in a device with a simple configuration not requiring addition of an A/D converter.